Accompanying the demand for higher integration levels and higher semiconductor device density is the increasing demand for cleanliness with respect to minute foreign objects on the surfaces of semiconductor wafers becomes. Consequently, in order to increase the manufacturing yield of semiconductor chips, it is necessary to remove foreign objects effectively by means of cleaning of semiconductor wafers.
Cleaning of semiconductor wafers includes multiple processes for removing minute particles made of many types of impurities, such as metal impurities, organic impurities, silicon, etc., from the surfaces of semiconductor wafers. In the cleaning process, the semiconductor wafers are cleaned sequentially in multiple cleaning solution tanks where various types of cleaning solutions are circulated. Multiple semiconductor wafers are held in any tray for transportation, and they are set in the cleaning solution tanks for prescribed cleaning times. In the following, the typical cleaning processing steps that are embodied at present will be shown.
(1) In order primarily to remove minute particles on the surfaces of semiconductor wafers, the semiconductor wafers are carried into a cleaning solution tank of SC-1 (ammonia/hydrogen peroxide aqueous solution: NH4OH/H2O2/H2O) and cleaned for 10 min.
(2) Then, in order to wash off SC-1 from the surfaces of the semiconductor wafers, the aforementioned semiconductor wafers are carried into a cleaning solution tank containing ultra-pure water and rinsed for about 10 min.
(3) Then, in order primarily to remove metal impurities and organic impurities on surfaces of the semiconductor wafers, the semiconductor wafers are carried into a cleaning solution tank of SC-2 (HCl/H2O2/H2O) and cleaned for about 10 min.
(4) Then, in order to wash off SC-2 from the surfaces of the semiconductor wafers, the aforementioned semiconductor wafers are carried into a cleaning solution tank containing ultra-pure water and rinsed for about 10 min.
(5) Then, in order primarily to remove silicon oxide film (natural oxide film) from the surfaces of semiconductor wafers, the aforementioned semiconductor wafers are carried into a cleaning solution tank of a hydrogen fluoride solution (diluted HF, FPM (HF/H2O2/H2O) cleaning solution, etc.) and cleaned for 1-5 min.
(6) As the last step, in order to wash off the HF (hydrogen fluoride) solution, the aforementioned semiconductor wafers are carried into a cleaning solution tank containing ultra-pure water and rinsed for 10 min.
However, in the aforementioned typical cleaning processes, minute particles cannot be easily removed. This is a problem. This is because in the aforementioned fifth step of operation, minute particles of silicon oxide film that peel off the surface of the semiconductor wafers become attached to the surface of the bare silicon. Then, in the sixth step of operation, the minute particles cannot be washed off. That is, with an acidic HF solution, the potential of the silicon wafer surface and that of the surface of the main minute particles are opposite in sign, so that they attract each other, and particle removal cannot take place by rinsing only with ultra-pure water.
In order to solve this problem, the conventional cleaning process adopts the following method: an ultrasonic vibrator is placed in the cleaning solution tank used in the aforementioned sixth step of operation, so that ultrasonic waves are irradiated for a prescribed time on the ultra-pure water or hydrogen-enriched ultra-pure water (ultra-pure water enriched with 0.3-1.6 ppm of hydrogen) while rinsing is performed. However, it is still difficult to remove the minute particles with this method.
Consequently, the purpose of the present invention is to provide a method that can effectively remove the minute particles adhering to the surface of semiconductor wafers.